The invention relates to a direct current to alternating current (DC/AC) power converter device optimized for variable-speed motors or for very high-speed motors for use in applications of the aviation compressor type, and also for applications of the aviation ventilation type.
The use of very high-speed motors, such as motors operating at more than 100,000 revolutions per minute (rpm) enables the size and the weight of the motor to be reduced significantly, thereby making it easier to integrate in equipment.
Nevertheless, the use of a motor of this type imposes major constraints on the electronic DC/AC converter controlling the motor, constraints that conventional control structures find difficult to address in an aviation environment.
Traditionally, a sinewave control inverter that is subjected to pulse width modulation control (DC/AC converter) is used for controlling the current in a variable-speed motor. Such control requires a switching or “chopping” frequency that is much higher than the electrical frequency of the motor, such as for example a chopping frequency that is ten to 25 times the electrical frequency of the motor. The electrical frequency is the product of the mechanical frequency of the motor multiplied by the number of pole pairs of the motor.
Controlling high-speed machines with sinusoidal control then requires the use of a power converter or “inverter” having a chopping frequency that is very high, thereby leading to a large number of technical challenges to be addressed, in particular relating to:                a very great increase in losses in power semiconductors;        better adapted traditional cooling means, thereby leading to additional difficulty in integrating the DC/AC converter; and        a technical break needed compared with present technology, given that a large band-gap semiconductor is needed and that isolated grid bipolar transistors (IGBTs) are not suitable.        
An alternative to sinusoidal control is to control the inverter with so-called “120°” control, thus making it possible to divide by six the number of switching operations performed by each switch, while maintaining the same chopping frequency. This is due to the fact that a single switch chops during ⅙th of the electrical period.
This kind of control improves switching losses in the power components, but degrades the quality of the current supplied to the motor, in particular because of the high level of harmonics, and leads to other constraints such as:                greater pulsation in the motor torque, with a risk of exciting resonant modes (shaft line, . . . );        degraded power factor and the appearance of harmonics at the inlet of the equipment; and        a risk of increasing the volume of the filters and of reinforcing mechanical parts on the shaft line.        
Furthermore, even with “120°” type control, the choice of chopping frequency remains associated with the electrical frequency of the motor.
Power conversion circuits are known in the prior art that comprise an inverter coupled downstream from a synchronous rectification DC/AC converter, also referred to as pulse amplitude modulation (PAM), which may be of the voltage lowering type (“buck”), of the voltage raising type (“boost”), or of the voltage lowering and raising type (“buck-boost”).
Such power converter circuits allow full-wave operation of the inverter and present the advantage of reducing losses in the converter circuit, and in particular in the inverter.
The circuit serves to dissociate the voltage-reducing function form the function of generating a stator frequency proportional to the mechanical frequency of the motor.
The role of the DC/DC converter is to impose a mean output voltage from the converter device, and thus, when the device is connected to a motor, to impose a mean voltage across the terminals of the motor in order to set its speed. Adjusting the output voltage of the buck DC/DC converter thus serves to control the speed of the motor.
An inverter, a DC/AC converter, serves to switch current through the phases of the motor at the electrical frequency of the motor. It does not modify the mean amplitude of the voltage of the motor.
Thus, when the converter device is connected to an electric motor, the device makes it possible to have a chopping frequency for the DC/DC converter that is independent of the electrical frequency of the motor to which the converter device is coupled.
The inverter operates with an inlet voltage that is controlled by the DC/DC converter and that is no longer directly subjected to voltage variations in the electricity network. This makes it possible to optimize the choice of power components in the inverter, and losses in the power semiconductors.
Nevertheless, such converter circuits generate additional harmonics in the motor and thus torque pulsation. This generation of additional harmonics can excite resonant modes and generate additional mechanical stresses.